Furnaces for use in heating the rooms of residential or commercial buildings are well known. Various types of fuel have been used for furnaces, including natural gas, propane and oil. Equally well known in the art, and recently introduced in the market place, are furnaces known as pulse-type furnaces. Such furnaces burn discrete charges of natural gas or propane, and frequently incorporate features which allow the furnace to operate at above 90% efficiency.
An example of a gas-burning pulse type furnace is disclosed in U.S. Pat. No. 4,568,265, issued to Mullen, et al. Such a furnace includes a combustion chamber into which slightly pressurized natural gas and air at atmospheric pressure are discretely introduced as successive charges resulting in the pulsating delivery of an air-fuel mixture. To start the furnace, air is initially forced into the combustion assembly by activating a purge fan and fuel is delivered by opening the natural gas valve. The first charge of the air-natural gas mixture is ignited by a spark ignitor located in the mixer head. As the first charge combusts, one-way flapper valves on the combustion air supply line and the natural gas supply line close, preventing flow of the exhaust gases thereinto, and forcing the flame front and expanding gases to move upwardly through the combustion chamber and out its exit into the tail pipe assembly. As the expanding gases depart the combustion chamber, a vacuum is formed therein which causes a sequential charge of air and fuel to be drawn into the combustion chamber. Due to the vacuum, this sequential charge does not require the purge fan to force combustion air into the chamber. Due to the heat generated by the combustion of the first charge, the successive charge ignites without the sparking of the ignitor. The furnace continues to operate by such pulsating combustion of successively introduced charges until such time that the fuel supply is turned off.
The outlet of the combustion chamber is connected to a tail pipe, whose length helps determine the frequency of the pulses. The tail pipe is formed in several loops extending from the combustion chamber and joins into an exhaust gas decoupler. From the decoupler, the exhaust gases are directed to a secondary heat exchanger disposed between a blower fan and the combustion chamber.
In order to heat the cold air returned from the living space, the blower fan draws air from the living space and forces the air first through the secondary heat exchanger, which causes the temperature of the exhaust gases flowing therethrough to drop below its dew point temperature. This causes the liquid vapor contained in the exhaust gases to condense, thereby transferring the vapor's latent heat of energy to the room air being heated. At the outlet of the secondary heat exchanger, the exhaust gases are approximately 130.degree. F. or less (i.e. generally below the dew point of the flue gas), and condensed liquid is discharged. After being initially heated by passing through the secondary heat exchanger, the room air to be heated is directed around the tail pipe, the exhaust gas decoupler, the combustion chamber, and the mixer head. The combustion chamber includes fins formed on its exterior surface parallel to the flow of air being heated. In this manner, heat is transferred to the room air from the air-fuel mixture combusting inside of the mixer head-combustion chamber, and from the movement of the flue gases through the tail pipe assembly, exhaust decoupler and secondary heat exchanger. The room air then exits the furnace, and is delivered to the living space being heated.
As mentioned above, in order to constrain the flow of the exhaust gases and flame front to one direction, one-way flapper valves are located on the combustion air supply and the natural gas supply. Additionally, there is an air-intake decoupler and a gas decoupler which are designed to minimize the noise produced by the furnace. The flapper valves are located upstream of the combustion assembly a distance sufficient to minimize the amount of heat transferred thereto. The operation of these flapper valves is well known in the industry.
As identified above, there are numerous sources of fuel which may be used with furnaces. However, there have been problems in developing pulse-type furnaces which operate on oil or other liquid fuels. Heretofore, the gas pulse-type furnace described above could not be modified to operate efficiently with oil or other liquid fuels. The problems encountered resulted from the significant differences between the combustion characteristics of liquid fuels and gaseous fuels. As used herein, gas fuel or gaseous fuels refers to any fuel burned in the gaseous state and not only to natural gas.
In addition to the lack of commercially successful and operational liquid fuel pulse furnaces, there are no pulse furnaces which can burn either liquid or gaseous fuels. There are numerous advantages of such a furnace. For example, a manufacturer would only need to manufacture, stock, and sell one type and size of furnace heat exchanger or heat train, and cabinet, which could be used by customers utilizing gaseous fuel as well as by customers utilizing liquid fuel for any particular range of heat input requirements. Particularly, one size heat exchanger and cabinet could cover a range of heating inputs (e.g. btu/hr. requirements) for both liquid and gaseous fuels, a larger heat exchanger and cabinet for a higher range of heating inputs, etc. Additionally, a customer who wished to switch from liquid fuel to gaseous fuel, or from gaseous fuel to liquid fuel can do so very simply and economically, without having to buy a new furnace. Equally advantageous as the furnace which may run on either gaseous fuel or liquid fuel without modifications is the pulse furnace which can be easily adapted to run on either type of fuel by installing certain minor parts designed for the specific type of fuel to be used, while all of the other components of the furnace remain the same, independent of the type of fuel used. A modular furnace (i.e. one in which the components that vary according to the type of fuel combusted are interchangeable with each other) in which the combustion assembly and heat train components remain the same, while the fuel delivery system is modular is particularly advantageous.
In particular, a major drawback has been the dramatic differences in designs between liquid fuel burning furnaces and gaseous fuel burning furnaces. External to the respective combustion assemblies, it is a relatively simple matter to adapt the fuel delivery system of the furnace to the particular type of fuel selected. However, the prior art combustion assemblies cannot be easily adapted to burn either liquid or gaseous fuels, and none of them can successfully burn liquid fuel in a pulsating combustion manner as described above. Clearly, there is a need for a pulse furnace that is easily and inexpensively adaptable to operate on either liquid fuels or gaseous fuels, depending upon the needs of the buyer. The simpler the adaptation of the furnace, and the greater the commonality of components, the easier and more practical it is for a manufacturer to stock and convert such furnaces, as well as providing corresponding reduced costs to the ultimate consumer and the ability to switch from one fuel to another.